Technical Abstract:
Cattle produce CH4 in the rumen and it represents a loss of feed energy. A possible cause of variation in feed efficiency may be differences in capacity to produce CH4. We hypothesized that cattle with a higher residual gain (RG) would have a lower abundance of methanogens in the rumen. Individual DMI and BW gain were determined on crossbred steers (n = 132, initial age = 348 ± 1 d and BW 444 ± 4 kg) for 56 d. Steers were offered feed ad libitum and individual intake was measured. The diet consisted of 82.75% rolled corn, 12.75% corn silage, and 4.5% supplement (0.066% monensin and 51% CP). Residual gain was calculated from the regression of BW on DMI; f(x) = (0.1262 ± 0.0128)x + (25.7 ± 9.9), R2 = 0.43. The 7 animals with the most extreme positive and negative RG that were within 32% of the STD of the mean DMI (772 ± 90 kg) were sampled. Steers were slaughtered and mixed rumen fluid was strained through cheesecloth and frozen. DNA was isolated from rumen content, and bacterial DNA was quantified using PCR with unique amplicons and is expressed as the log of the DNA concentration. Total archaea bacteria standardized to total bacterial DNA (16S) did not differ between High and Low RG (P = 0.96). High and Low RG steers did not differ for Methanobrevibacter ruminantium+ Mbb. cuticularis (P = 0.56), Methanosarcina barkeri (P = 0.58), or Methanobacterium ruminantium (P = 0.54) after standardizing for total archaea bacteria. The concentration of Mbb. smithii+wolinii+thaueri+gottschalkii+ woesii tended to be greater in the High RG steers (P = 0.06). While species composition may shift between cattle with different RG, total methanogens did not differ suggesting that differences in feed efficiency may not be a function of rumen microbial capacity to produce methane. Partially funded by National Institute of Food and Agriculture Grant 2011-68004-30214 National Program for Genetic Improvement of Feed Efficiency in Beef Cattle.